To meet the demand for higher integration density and operating speed of LSIs, the effort to reduce the pattern rule is in rapid progress. The wide-spreading flash memory market and the demand for increased storage capacities drive forward the miniaturization technology. As the advanced miniaturization technology, manufacturing of microelectronic devices at the 65-nm node by the ArF lithography has been implemented in a mass scale. Manufacturing of 45-nm node devices by the next generation ArF immersion lithography is approaching to the verge of high-volume application. The candidates for the next generation 32-nm node include ultra-high NA lens immersion lithography using a liquid having a higher refractive index than water in combination with a high refractive index lens and a high refractive index resist film, extreme ultraviolet (EUV) lithography of wavelength 13.5 nm, and double patterning version of the ArF lithography, on which active research efforts have been made.
With respect to high-energy radiation of very short wavelength such as electron beam (EB) or x-ray, hydrocarbons and similar light elements used in resist materials have little absorption. Then polyhydroxystyrene base resist materials are under consideration.
The exposure system for mask manufacturing made a transition from the laser beam exposure system to the EB exposure system to increase the accuracy of line width. Since a further size reduction becomes possible by increasing the accelerating voltage of the electron gun in the EB exposure system, the accelerating voltage increased from 10 keV to 30 keV and reached 50 keV in the current mainstream system, with a voltage of 100 keV being under investigation.
As the accelerating voltage increases, a lowering of sensitivity of resist film becomes of concern. As the accelerating voltage increases, the influence of forward scattering in a resist film becomes so reduced that the contrast of electron image writing energy is improved to ameliorate resolution and dimensional control whereas electrons can pass straightforward through the resist film so that the resist film becomes less sensitive. Since the mask exposure tool is designed for exposure by direct continuous writing, a lowering of sensitivity of resist film leads to an undesirably reduced throughput. Due to a need for higher sensitivity, chemically amplified resist compositions are contemplated.
As the feature size reduces, image blurs due to acid diffusion become a problem. To insure resolution for fine patterns with a size of 45 nm et seq., not only an improvement in dissolution contrast is important as previously reported, but control of acid diffusion is also important as reported in Non-Patent Document 1. Since chemically amplified resist compositions are designed such that sensitivity and contrast are enhanced by acid diffusion, an attempt to minimize acid diffusion by reducing the temperature and/or time of post-exposure baking (PEB) fails, resulting in drastic reductions of sensitivity and contrast.
A triangular tradeoff relationship among sensitivity, resolution, and roughness has been pointed out in Non-Patent Document 2. Specifically, acid diffusion must be suppressed in order to enlarge the exposure margin whereas a short acid diffusion distance of less than 50 nm leads to a rapid degradation of roughness.
The addition of an acid generator capable of generating a bulky acid is an effective means for suppressing acid diffusion. It was then proposed to incorporate in a polymer an acid generator of an onium salt having a polymerizable olefin. Patent Document 1 discloses a sulfonium salt having polymerizable olefin capable of generating a specific sulfonic acid and a similar iodonium salt. Patent Document 2 discloses a sulfonium salt having sulfonic acid directly attached to the main chain.
A tradeoff relationship between sensitivity and roughness has been pointed out. For example, Non-Patent Document 3 describes that sensitivity is in inverse proportion to roughness. It is expected that the roughness of a resist film is reduced by increasing the exposure dose to reduce shot noise. Non-Patent Document 4 describes a tradeoff between sensitivity and roughness in the EUV lithography in that a resist material containing a more amount of quencher is effective in reducing roughness, but suffers from a decline of sensitivity at the same time. There is a need to enhance the quantum efficiency of acid generation in order to overcome the problem.
With respect to the acid generating mechanism triggered by EB exposure, Non-Patent Document 5 reports that PAG releases acid through the mechanism that a polymer is excited by exposure so that electrons migrate to the PAG. Since the irradiation energy of EB or EUV is higher than the threshold value (10 eV) of ionization potential energy of a base polymer, it is presumed that the base polymer is readily ionized. An exemplary material of accelerating electron migration is hydroxystyrene.
It is reported in Non-Patent Document 6 that poly-4-hydroxystyrene has a higher acid generation efficiency in EB exposure than poly-4-methoxystyrene, indicating that poly-4-hydroxystyrene provides for efficient migration of electrons to PAG upon EB exposure.
Patent Document 3 discloses a resist material comprising a base resin having a sulfonium salt bound thereto wherein the sulfonium salt is liberated under the action of the generated acid. This resist material is less effective in reducing roughness (LWR) and has room for a further improvement in sensitivity.
Reported in Non-Patent Document 7 is a material obtained through copolymerization of hydroxystyrene for increasing the acid generation efficiency by electron migration, a methacrylate of PAG having sulfonic acid directly bonded to a polymer backbone for suppressing acid diffusion, and a methacrylate having an acid labile group. This material is still insufficient. A material having satisfactory sensitivity, resolution and roughness is desired.